Magnetic recording techniques have a number of excellent characteristics which are not provided by other recording systems, such as the ability to repeatedly use the recording medium, ease of conversion of input data into an electronic signal, the ability to combine the recording technique with relevant equipment to form a system, and the ability to readily process a signal. Accordingly, magnetic recording techniques have been extensively applied to various fields such as the video, audio and computer fields. In order to meet the demand for miniaturization of equipment, enhancement of recorded and reproduced signals, the trend to longer recording, and an increase in recording capacity, further improvement in recording density is needed. For a coating type magnetic recording disk, various means have been proposed in which the particle size of the ferromagnetic powder is decreased, the dispersibility thereof is improved, and the packing density in a magnetic layer is increased. In another effective means, a ferromagnetic metal powder and a hexagonal ferrite having excellent electromagnetic characteristics have been used.
The widespread use of OA equipment (i.e., Office Automation Equipment) such as a minicomputer and a personal computer has been accompanied by a marked increase in the popularity and use of magnetic recording disks. A magnetic recording disk is used and stored under broad environmental conditions of temperature and humidity and in dusty environments. But an improvement in recording density is strongly desired in order to achieve a large data recording capacity and miniaturization of the recording disks. In order to obtain a magnetic recording disk suitable for high density recording with an acicular ferromagnetic powder as in the past, it was necessary to employ a maximum size of the acicular ferromagnetic powder sufficiently smaller than the recording wavelength or a record bit length. At present, an acicular ferromagnetic powder having a size of 0.3 .mu.m has already been put to practical use, which makes it possible to record at wavelengths of 1 .mu.m or less.
It is necessary to further reduce the size of the acicular ferromagnetic powder in order to obtain a medium which enables even higher density recording. However, such small-sized acicular ferromagnetic powder is disadvantageous in that the thickness thereof, which is 100.ANG. or less, is very fine and the particle volume thereof, which is about 10.sup.-17 cm.sup.3, is very small. Consequently, the electromagnetic characteristics thereof are reduced by thermal disturbance and surface effects, so that sufficient orientation cannot be obtained by applying a magnetic field to a coated magnetic layer.
In recent years, a high recording density magnetic recording medium has been developed based on a hexagonal series ferrite powder which is tabular and has an axis of easy magnetization in the perpendicular direction to a plate, as a ferromagnetic powder as described, for example, in U.S. Pat. No. 4,425,401 corresponding to JP-A-58-6525 (the term "JP-A" as used herein means an unexamined published Japanese patent application) and JP-A-58-6526. This ferromagnetic powder allows for an average particle size of 0.05 .mu.m and high density recording.
Furthermore, a narrow track width is required for high density recording. In order to satisfy these requirements, the development and application of a magnetic disk comprising ferromagnetic metal powder and ferromagnetic hexagonal series ferrite for size miniaturization and improvement in recording density have been intensively investigated. In particular, the shift towards a thinner magnetic layer and high output are desired for achieving a high recording density and an improved overwriting electromagnetic characteristic. However, there is a concern that the shift towards a thinner magnetic layer might be accompanied by extreme deterioration of running durability.
An overwrite of the recording signals with different magnetic wavelengths is usually necessary in a magnetic recording disk for a computer such as a floppy disk. It was sufficient in the past to carry out the overwrite of two kinds of signals, 1f and 2f, which are in a relationship of two times in terms of frequency. However, not only a shorter recording wavelength, but also an overwrite of a plurality of RLL signals with a frequency ratio of 3:8 present at a broader range, are required for a magnetic recording disk with a high capacity of 10M bites, in which high capacity has been strongly desired in recent years. Where a signal having a short recording wavelength and a large difference in recording frequency is used, improvement in the electromagnetic characteristics of the magnetic layer was the only limiting factor for successfully overwriting a signal with a short recording wavelength on a signal with a long recording wavelength as disclosed in U.S. Pat. No. 4,788,092 corresponding to JP-A-58-122623 and U.S. Pat. No. 4,895,758 corresponding to JP-A-61-74137.
In conventional magnetic layers with a thickness of 1.0 .mu.m or more, overwrite of a signal with a shorter wavelength on a formerly recorded signal with a longer wavelength cannot erase the formerly recorded signal since a line of magnetic force cannot reach through the entire depth of the magnetic layer.
Furthermore, an improvement in recording density is accompanied by a narrower gap between the recording heads, which causes difficulty in sufficiently recording in the thickness direction of the medium.
In order to solve the above problem, a thin magnetic layer of 1 .mu.m or less was proposed. However, the thin magnetic layer was liable to peel off and good running durability (a main factor for preventing dropout) could not be obtained, thus deteriorating reliability.
Accordingly, in developing a magnetic recording disk having the desired high density recording, improvement in reproducing output, securing an overwriting characteristic and running durability, in particular, have become obstacles.
Electrification in running a magnetic recording disk increases the number of dropouts attributable to adherence of dust, and the error rate thereby has become a fatal defect. In order to solve this electrification problem, methods are employed in which an additive is added to prevent electrification in the magnetic layer. Among them, the method in which carbon black is added is the most effective and broadly applied. However, in the above magnetic recording disk for high density recording, the addition of carbon black lowers the packing degree of the magnetic substance to deteriorate the output. Therefore, the addition amount of the carbon black is limited, so that the antistatic effect is insufficient.
Various proposals for preventing electrification and providing high output and improved durability are disclosed in JP-A-55-55431, JP-A-55-55432, JP-A-55-55433, JP-A-55-55434, JP-A-60-164926, JP-A-55-55436, JP-A-62-38523, and JP-A-62-159337.
Namely, an intermediate layer is provided between a magnetic layer and a support, wherein the intermediate layer containing carbon black and a binder resin is coated and then the magnetic layer is formed thereon.
However, it has not been possible to achieve even with this method, excellent electromagnetic characteristics, that is, a high reproduction output, an overwrite characteristic and a sufficient running durability in the above high density recording magnetic recording medium.
In order to obtain high capacity and high output electromagnetic characteristics, it is proposed to make the magnetic layer thinner, for example, in the following examples.
1) JP-A-57-198563 (the term "JP-A" as used herein means PA0 2) JP-A-62-154225: PA0 3) JP-A-63-187418: PA0 4) JP-A-60-93631: PA0 5) JP-A-61-57036: PA0 6) JP-A-58-85931:
an unexamined published Japanese patent application):
There is disclosed an attempt in which a resin layer containing no fine powders is previously coated under a magnetic layer and then an extremely thin magnetic layer is coated to reduce a self-demagnetization.
With this method, however, the surface property of the magnetic layer is not sufficient, and further the problem that the increase in the thickness of the resin layer accelerates curling due to shrinkage of the resin occurs.
There is proposed a method in which the thickness of the non-magnetic layer is increased more than that of the magnetic layer to decrease the relative thickness of the magnetic layer. The yield in this magnetic recording medium is very inferior and the manufacturing process thereof is complicated. Accordingly, there is a problem in the practicality thereof.
There is described a method in which a lower nonmagnetic layer and an upper magnetic layer are simultaneously coated while wet to thereby obtain a magnetic layer of 2 .mu.m or less. According to this method, the magnetic layer can be thinned by as much as 1 .mu.m or less and in addition, a magnetic layer with an excellent surface property can be obtained, whereby the magnetic recording layer having relatively excellent electromagnetic characteristics can be obtained. However, even this method provides electromagnetic characteristics which are still insufficient as compared with those of a thin layer type magnetic recording medium, and the requirement for an increase in recording density in the future will not be met.
Further, there have been disclosed various means in which abrasive particles are used in order to improve running durability. The examples thereof are described below.
It is disclosed to use abrasive particles having a presence density of 20 particles or more per 100 .mu.m.sup.2 on a magnetic layer surface and a particle size of more than 1.5 times or less as large as that of the ferromagnetic powder. This is effective for a video tape, but running durability can not be provided at all in the magnetic recording disk.
It is disclosed that 0.25 particle/.mu.m.sup.2 or more density is required for the abrasive on a surface. However, when such a large amount of the abrasive is present, durability is improved but the electromagnetic characteristics are not suited to a high density recording.
It is disclosed to use two kinds of abrasives, each having a different particle size. However, it is difficult to obtain a durability which is compatible with high output in a disk medium such as a floppy disk, simply by use of two kinds of abrasives each having a different size for a high recording density medium.
As described above, it is difficult to obtain a running durability which is compatible with the electromagnetic characteristics, and an effective solution to this requirement is desired.